1. Field of Invention
The present invention relates to a pretreatment method for the surface of steel T91/P91, especially to a pretreatment method for improving antioxidation of steel T91/P91 in high temperature (500-750° C.) water vapor.
2. Description of Related Art
Currently, due to the excellent performance, the series ferritic steel containing 9-12% of Cr is applied to large-diameter P91 vapor pipes (main vapor pipes and reheat vapor pipes) and small-diameter T91 vapor pipes (superheater pipes and reheater pipes), which are used for thermal power generation. Compared with the conventional ferritic steel, such material has a better mechanical property and thus can be applied at a higher temperature and pressure, thereby improving the efficiency of thermal power generation. Currently, steel T91/P91 has become a common material used in the supercritical units of a power station boiler, due to relatively high tensile strength, high temperature creep and endurance strength, low thermal expansibility, excellent thermal conductivity, malleability and antioxidation capability, as well as high tenacity. However, the steel T91/P91 may still be seriously oxidized in high temperature and high pressure water vapor after being oxidized for a long time or when operating at a higher temperature.
In a water vapor atmosphere, at 500-750° C., the oxidation speed of the steel T91/P91 greatly increases as the temperature rises. The oxidation product of the steel T91/P91 comprises Fe2O3, Fe3O4 and (Fe,Cr)3O4. Since such material contains low content of Cr, no continuous or compact Cr2O3 layer is formed in oxide films generated at different temperatures, and even no Cr2O3 phase is formed in the oxide films. In general, the oxidation product is in a form of (Fe,Cr)3O4 solid solution. As the oxidation speed increases and the temperature changes, a thicker oxide film is subjected to larger growth stress and thermal stress, so the plastic deformation of the oxide film is limited. Thus, obvious oxide film stripping occurs in application of such materials, and in turn the oxide film stripping further increases the oxidation speed.
When the steel T91 and steel P91 are applied to a vapor pipe for thermal power generation, applying a coating on the inner wall or modifying the surface is one of efficient ways for improving the antioxidation of the steel T91 and steel P91 in the high temperature water vapor. However, the process for applying a coating/plating layer in a small-diameter vapor pipe is generally complicated, and in a simpler hot-dip aluminum plating process, due to the brittle phase of a Fe—Al intermetallic compound generated in the process, the plating layer is stripped during oxidation, and meanwhile, the mechanical property of the pipe is greatly influenced.
T. Sundararajan [T. Sundararajan, et al: Surface and Coatings Technology, 2006, 201, 2124.] detected the oxidation behavior of a sample in water vapor at 650° C. directly after nano-CeO2 was coated on the surface of steel T91. The result indicates that the oxidation speed is lower than that of a blank sample. However, after being oxidized for 500 hours, the external layer of the oxide film is a ferric oxide, and the inner layer is a film composed of mixed oxides of Fe, Cr, and Si, whose antioxidation capability in water vapor is still limited.
Li Xingeng and Wang Xuegang [Li Xingeng, Wang Xuegang, et al: Corrosion Science and Protection Technology, 2008, 20(3) 157-161.] researched the oxidation behavior of a CeO2 film deposited on the surface a Fe—Cr alloy containing 9% of Cr in the water vapor at 600-770° C. The result indicates that the deposited rare earth thin film neither changes the structure of the oxide film nor obviously reduces the oxidation speed.